This article investigates the reasons behind the disappearance of Francis Glisson's theory of irritability during the eighteenth century. At a time when natural investigations were becoming increasingly polarized between mind and matter in the attempt to save both man's consciousness and the inert nature of the res extensa, Glisson's notion of a natural perception embedded in matter did not satisfy the new science's basic injunction not to superimpose perceptions and appetites on nature. Knowledge of nature could not be based on knowledge within nature, i.e., on the very knowledge that nature has of itself; or--to look at the same question from the point of view of the human mind--man's consciousness could not be seen as participating in forms of natural selfhood. Albrecht Haller played a key role in this story. Through his experiments, Haller thought he had conclusively demonstrated that the response given by nature when irritated did not betray any natural perceptivity, any inner life, any sentiment interiéur. In doing so, he provided a less bewildering theory of irritability for the rising communities of experimental physiology.